Do houses have gardens? Or is it also a situation where they rent or own a plot of land for gardening on the outskirts of the city, like for instance Vienna?

Most houses have a garden. Even if that are attached in a long row sharing walls (semi-detached) and ground floor. In the city, apartments would not have and I have not seen any common gardens. Housing in Brussels is relatively affordable. This has a negative impact on congestion and it is a nightmare getting around the city.